What I don't understand is why the joint return threshold is not twice what it is for a single person.

If
a single person has a allowable Adjusted Gross Income of $200,000
before the 3.8 percent surtax may apply, why wouldn't a couple filing a
joint return have a threshold of $400,000? Instead, the couple starts
paying this at only $250,000.

Is Congress suggesting a way for high-income taxpayers to save tax is that they get divorced and file as single persons?